In this investigation I want to look into how a thermistor works, then experiment to find how temperature change affects its resistance. The main aim of this investigation is to design and build, using a thermistor, a temperature sensing circuit.

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Introduction

Thermistor Investigation Aim: In this investigation I want to look into how a thermistor works, then experiment to find how temperature change affects its resistance. The main aim of this investigation is to design and build, using a thermistor, a temperature sensing circuit. This will be for use in a computer system. Because of the recent advances in the speeds of PCs there has been a big problem in to deal with the extra heat that their faster CPUs (central processing units) produce. Most manufactures have simply increased the surface area of heat sinks used and fitted larger and more powerful fans but even these are sometimes not enough. If the same air is being forced through a heat sink over and over again it becomes quite warm and unable to remove more heat from the CPU so its temperature will rise. To solve this problem case fans are needed to refresh the air in the case but these can be noisy. Because the amount of heat produced varies and there is a big difference between the heat made on idle and when the CPU is under load (it can vary from 30w to 100w in some cases). This means the extra case cooling would only be needed when the CPU is under load and reaches a certain temperature. Noise is that last thing you want when trying to concentrate at a computer so by only having the case fans on when needed to remove ...read more.

Middle

I will use this to turn on a relay when the temperature is high enough and the relay will switch on the fan. Here is my component list: Thermistor Rheostat Power supply x2 Cables Relay 80mm case fan Multimeter - to help set up the resistance of the rheostat. The relay switches at 3v and the power supplies will mimic the power supply in a computer for this prototype. This means I have the choice of 2 input voltages: 5v and 12v (as is standard for an ATX Power supply). For the potential divider I will use 5v because 12v is un-necessary but I will power the fan with 12v because that is its rated voltage. The relay also has the ability to work in two ways - to break the circuit when powered or to make it. For this circuit I will use it to break. I am using a rheostat instead of a fixed resistor because it gives me the opportunity to adjust the temperature at which the fan will come on. This is useful for using the design on different processors and in different situations as the safe temperature for different CPUs varies. I will set up my circuit as shown in fig.4 The thermistor would be placed as close to the core of the CPU as possible to get the most accurate readings. ...read more.

Conclusion

the water wasn't that high there was practically no chance of anything going wrong because it takes at least 12v for water to conduct electricity. If I had more time I would firstly like to test the system in a real PC to see if it still works as well and to check that the fan has the desired effect on the computer temperature. In a real life situation the relay could turn on multiple fans for extra cooling power. Also I would like to make a numbered dial for the rheostat so that it can be adjusted to switch the fan on at a desired temperature without having to set up the rheostat with a ohm meter. My circuit could also be applied to other applications that involve cooling such as turning on fans to cool rooms etc. It could also be reversed and used to turn on a heater if something gets too cold. Fig.2 Temperature (�C) Test 1 (Ohms) Test 2 (Ohms) Test 3 (Ohms) Test 4 (Ohms) Average (Ohms) 75 147 152 174 155 157 70 173 174 195 178 180 65 198 204 215 207 206 60 230 240 229 233 233 55 272 279 280 283 278.5 50 315 324 327 324 322.5 45 364 377 375 371 371.75 40 447 448 458 450 450.75 35 580 595 615 591 595.25 30 714 720 711 716 715.25 25 860 872 852 870 863.5 20 1130 1135 1157 1148 1142.5 Fig.3 ...read more.

Background theory: Lemon batteries: A small charge can be produced by putting two dissimilar metal electrodes in either side of a lemon, and joining it into a circuit, the charge produced is small but to give the largest voltage possible the electrodes need to be largely different in electrode potential.

Preliminary experiment: In order to make sure that my experiment would be as accurate and as successful as it could be I conducted a preliminary experiment. In my preliminary experiment I found out a few potential problems. One of these was the fact that when the temperature of the water

the conduction band is by gaining enough energy to jump the gap. In metals the forbidden gap between the bands is extremely small and so it takes an extremely small amount of energy for the electrons to jump the gap as so electricity can flow very easily through all metals - this is why they conduct so well.

When the temperature falls to 90 C, then the thermistor should then be placed into the beaker, sunken underwater. Because there is a danger with the interaction of water and electricity, it is important to either wear gloves or use a non-conducting material to hold the wires to the edge of the beaker.

Diagram This scientific diagram shows how we set up our equipment. Fair Testing To ensure that this experiment will be unbiased we will use the same equipment throughout the whole test, the same volume of water and only vary two factors; the temperature and the number of batteries in the circuit.

The final temperature achieved at different points within the aluminium will depend on how the block is lagged. If the heat escapes through the sides then the greater the temperature of the block, the greater the temperature gradient and therefore the more heat is lost.